【转帖】can a profile tolerance used to control a true position

yang686526

can a profile tolerance used to control a true position?
a current production cam cover has 11x mounting holes. this holes true position is not in drawing, instead we got a 1.0mm profile tolerance to control this position... can this be done? it is a mistake? thanks
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it's perfectly legal per asme y14.5m-1994 although whether it's most appropriate based on function, other users of the drawings etc. may be an issue.
profile tolerance controls size, form & location.
kenat,
is the cam cover a casting or molding and are the holes "as cast or as molded"?
paul
jcastaneda79,
   if a profile tolerance is applied to a hole, the thing can be interpreted as per asme y14.5m-1994, and inspected.  i have even done this on one or two occasions.
   whether it is good practise, or the optimal way to dimension your drawing, is more open to question.
   i like to apply to holes zero positional tolerances at mmc.  an alternative to this would be to apply a unilateral profile tolerance with the guide line outside the hole.  see figure 6.5.1 in the standard.  
   either way, you are clearly defining an mmc condition, which is the important thing for a clearance hole.
               jhg
the cam cover is as cast, so in this case, the 1.0 mm can be interpreted as i can move the hole′s center 0.5 to one side and 0.5 to the other side? thanks
jcastaneda79,
that's not the correct interpretation.  the 1.0 mm profile tolerance doesn't directly control the hole's center, it controls the hole's surface.
the tolerance zone is a tube-shaped shell 1.0 mm thick, straddling the nominal surface of the hole.  the od of the tube will be the hole's basic diameter plus 0.5 mm, and the id of the tube will be the basic diameter minus 0.5 mm.
in this case, the profile zone controls the size, form, orientation and location of the hole.
evan janeshewski
axymetrix quality engineering inc.
drawoh,
if one was to relate a variable position tolerance to a profile tolerance of a hole... the position tolerance would have to be zero @ mmc, zero @ lmc, and maximum at nominal.
see the attached paper.
paul   
pauljackson,
   i agree that the two dimension conditons i described are not identical.  in most cases, the mmc state is much more important than the others.  
   the accurate position at lmc might be what i actually want!  think about a hex socket head cap screw clamping down on the maximum sized hole.
               jhg
jcastenida79:
firstly, are the diameters of the holes in question shown in a basic dimension? if they are not, then i think we may have a design error here since the true profile of each hole should be basic. i would then ask for clarification from the designer.
please suggest that holes that have a function or relationship with the mating part should be shown in positional rather than with profile of a surface and also suggest at mmc. it just is more appropriate and as you can see from some of the answers, it can get a bit problematic using profiles.
dave d.
jcastenida,
there is nothing wrong with having the cored holes of the casting toleranced the same as the cast surface contour.
dave is right that if the hole sizes are toleranced rather than basic then there is a problem... but... i don't know what would lead him to suspect that?
the only thing "a bit problematic" as dave puts it, about measuring cored hole's is that people typically want to see the cored hole's size and coordinate location data rather than its contour points data. not that it would do them much good in modifying the mold contour or core pin sizes to achieve contour conformance post shrink.
once they have that size, x and y data they are perplexed about how to relate it to the profile tolerance. i have seen quite a few negotiated substitute specs to constrain size and position but none of them captured the liberties that size and location have within the profile boundaries. all of them proportioned the profile tolerance into fixed amounts for size and either fixed or variable amounts for position. that is why i created that paper to show how to figure a position tolerance for any measured core feature size.
paul  

i would stick to the recommendation in paragraph 2.1.1.1, and only use position for features of size like round holes.
axym (evan j.) has a good point in noting that you are controlling the tubular tolerance zone of the holes and ignoring the centering of the feature as position would do